1. Field of the Invention
The present invention relates to an optical pickup and an optical disc apparatus which record and/or reproduce information from an information recording medium such as an optical disc.
2. Description of the Related Art
In order to record and/or reproduce information from an information recording medium such as an optical disc by an optical pickup, it is necessary to correctly apply a convergence spot onto a predetermined recording track on the optical disc.
In order to correctly apply the convergence spot onto the recording track on the optical disc, a differential push-pull method (hereinafter, it is called a “conventional DPP method”) is traditionally widely used as a means for detecting a tracking error signal.
An optical pickup using the conventional DPP method has a light source which emits a light beam of a predetermined wavelength, and an objective lens which converges the light beam emitted from the light source onto the signal recording surface of an optical disc 11, in which between the light source and the objective lens, a diffractive element 205 is disposed which has a grating pattern with a projected and recessed pattern as shown in FIG. 14, and the diffractive element 205 splits the light beam emitted from the light source into three light beams formed of a zero order diffracted beam and positive and negative first order diffracted beams.
As shown in FIG. 15, these three light beams are converged into the objective lens to form three convergence spots having a main spot 200 which is formed by converging the zero order diffracted beam onto the optical disc 11 and first and second sub-spots 201 and 202 which are formed by converging the positive and negative first order diffracted beams.
As shown in FIG. 15, the first and second sub-spots 201 and 202 are converged at a position shifted by ½ Tp relatively in the track pitch direction (the tracking direction) of the optical disc 11 with respect to the main spot 200 (here, Tp represents a track pitch).
Then, the wavefront shape and the light intensity distribution of the spots 210 and 211 on light receiving surfaces 34a and 34b of a photodetector which detects the returning light from the optical disc 11 area shown in FIGS. 16A to 16D, and the light and dark conditions of a main spot 210 and a sub-spot 211 are inverted, the main spot 210 is received on the light receiving surface which receives the zero order diffracted beam reflected at the main spot 200 on the optical disc 11 and the sub spot 211 is received on the light receiving surface which receives the positive first order diffracted beam reflected at the first sub-spot 201. In addition, the light and dark conditions of a sub-spot 212 are the same as those of the sub-spot 211, the sub-spot 212 is received on a light receiving surface 34c which receives the negative first order diffracted beam reflected at the second sub-spot 202 on the optical disc 11. Therefore, the push-pull operation output of the main spot 210 on the light receiving surface and the push-pull operation outputs of the sub-spots 211 and 212 on the light receiving surface are differentially operated to obtain a tracking error signal.
However, in the optical pickup according to the conventional DPP method, as described above, it is necessary to set the spacing between the main spot 200 and the first and second sub-spots 201 and 202 on the optical disc 11 in the tracking direction to about one half of the track pitch Tp. Thus, there is a problem that an excellent tracking error signal may not be obtained from optical discs 11 having different track pitches, for example, a DVD±R disc and a DVD-RAM disc.
In order to solve the problem, Japanese patent No. 3549301 and JP-A-2004-145915 (Patent References 1 and 2) describe a method which can obtain a tracking error signal from optical discs having different track pitches (hereinafter, it is called an “in-line DPP method”).
An optical pickup using the method described in Japanese patent No. 3549301 (hereinafter, it is called a “two area in-line DPP method”) has a two area diffractive element 225 which is split into two areas 225a and 225b as shown in FIG. 17, in which the diffractive element 225 splits a light beam emitted from a light source into three light beams formed of a zero order diffracted beam and positive and negative first order diffracted beams. In addition, the phases of the periodic structures of the first and second areas 225a and 225b of the diffractive element 225 are formed to be varied at an angle of 180 degrees.
As shown in FIG. 18, the three light beams split by the diffractive element 225 are converged into the objective lens to form three convergence spots formed of a main spot 220, a first sub-spot 221 and a second sub-spot 222 on an optical disc 11. As shown in FIG. 18, the first and second sub-spots 221 and 222 are arranged on the same track as the main spot 220.
As described above, since the phases of the periodic structures of the first and second areas 225a and 225b of the diffractive element 225 are varied at an angle of 180 degrees, even though the three convergence spots 220, 221 and 222 are arranged on the same track as shown in FIG. 18, the wavefront shape and the light intensity distribution of light spots 230, 231 and 232 on a photodetector area shown in FIGS. 19A to 19D, and the light and dark conditions of the main spot 230 and the sub-spots 231 and 232 on the light receiving surface are inverted. In addition, FIGS. 19C and 19D show the light intensity distribution and the wavefront shape of the sub-spot 231 on the light receiving surface which receives the positive first order diffracted beam, and the light and dark conditions of the sub-spot 232 on the light receiving surface which receives the negative first order diffracted beam are also the same.
Therefore, a tracking error signal can be obtained by an identical operation with that of the conventional DPP method with the three convergence spots 220, 221 and 222 arranged on the same track. Thus, an excellent tracking error signal can be obtained from each of the optical discs having different track pitches.
However, in the optical pickup according to the two area in-line DPP method, there is a problem that the displacement of the objective lens is displaced to greatly decrease the push-pull operation outputs of the sub-spots 231 and 232 on the light receiving surface. This is because the objective lens is displaced to make the light intensity distribution and the wavefront shape of the sub-spot 231 as shown in FIGS. 20A and 20B, generating an area in which the light and dark conditions are identical with those of the main spot 230. In addition, for the sub-spot 232, its light and dark conditions are similarly varied as those of the sub-spot 231.
On the other hand, in an optical pickup according to the method described in JP-A-2004-145915 (hereinafter, it is called a “three area in-line DPP method”), a three area diffractive element 245 is used which is split into three areas as shown in FIG. 21 in order to solve the problem of the two area in-line DPP method described above. More specifically, the diffractive element 245 has first to third diffraction areas 245a, 245b and 245c in which the phases of the periodic structures of the first and third diffraction areas 245a and 245c are varied at an angle of 180 degrees as similar to the two area type as described above, but the phase of the periodic structure of the second diffraction area 245b arranged between the first and third diffraction areas 245a and 245c is varied at an angle of 90 degrees with respect to the first and third diffraction areas 245a and 245c each.
The diffractive element 245 splits the light beam emitted from the light source into three light beams formed of a zero order diffracted beam and positive and negative first order diffracted beams. As similar to the case according to the two area in-line DPP method shown in FIG. 18, the three split light beams form three convergence spots formed of a main spot 220, a first sub-spot 221 and a second sub-spot 222 converged onto the optical disc 11 by an objective lens.
As described above, the second area 245b is arranged between the first and third areas 245a and 245c, whereby the wavefront shape and the light intensity distribution of a sub-spot 241 on the light receiving surface which receives the positive first order diffracted beam area shown in FIGS. 22A and 22B, and the sub-spot 241 has an area in which the light and dark conditions are halfway inverted with respect to the main spot 240, that is, it has an area of an intermediate condition between the light part and the dark part. In addition, the light intensity distribution and the wavefront shape of the main spot 240 are the same as those of the main spot 230 shown in FIG. 19A as described above. Moreover, the light and dark conditions of a sub-spot 242 on the light receiving surface which receives the negative first order diffracted beam are the same as those of the sub-spot 241.
In the optical pickup according to the three area in-line DPP method, when a displacement of the objective lens is equal to or below a predetermined amount ΔX1, as shown in FIGS. 23A and 23B, the ratio of the semi-inverse area of the sub-spot 241 on the light receiving surface is increased, but such an area is not generated that the light and dark conditions are identical with those of the main spot 240.
In the optical pickup according to the three area in-line DPP method, because of the existence of the semi-inverse area, the push-pull operation outputs of the sub-spots 241 and 242 drop, but a decrease is smaller than that in the case having an area in which the light and dark conditions are identical with those of the main spot 240. Thus, a tracking error signal can be obtained that the characteristics of the field of view are more excellent than that by the two area in-line DPP method.
However, the optical pickup according to the three area in-line DPP method has the following problem. More specifically, as shown in FIGS. 24A and 24B, under the condition in which a displacement of the objective lens is ΔX2 that is greater than a predetermined amount (ΔX1), an area having the light and dark conditions identical with those of the main spot 240 is generated in the sub-spot 241 on the light receiving surface. Therefore, when a displacement of the objective lens is equal to or greater than a predetermined amount ΔX1, a decrease of push-pull computation outputs of the sub-spots 241 and 242 is almost the same as that of the two area in-line DPP method. In the three area in-line DPP method, in order to increase a displacement of the objective lens at which the first and second sub-spots 241 and 242 begin to have an area in which the light and dark conditions are identical with those of the main spot 240, it is sufficient that the width of the second area 245b of the diffractive element 245 is increased. However, on the other hand, when the width of the second area 245b is increased, a problem arises that the push-pull operation output of the sub-spot on the light receiving surface drops under the condition that the objective lens is not displaced.
More specifically, in the three area in-line DPP method, when the suppression of the decrease in tracking error is as well intended in the range of a wide displacement of the objective lens, a problem arises that the signal-to-noise ratio of the tracking error signal itself is deteriorated.